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What is the difference between Var, Let and Const in JavaScript (ES6), -…

- - - - i.e.
        
        var tester = "Hi";
        function newFunction() {
        var hello = "hello!";
        }
        console.log(hello);
        error: helllo is undefined.
    - - var greeter = "hey hi";
        var times = 4;
        if (times > 3) {
        var greeter = "say hello instead";
        }
        console.log(greeter); => "say hello instead"
        
        Since times > 3 returns true, redefine greeter to saysay Hello. This code is problematic if you have redefined greeter on purpose, but it becomes problematic when you don't know that the variable greeter has been defined before.
        在local function scope内因为重新申明了变量所以替换了之前在global scope的变量，这也会导致无意识的错误发生
  - - - let greeting = 'say Hi';
        let times = 4;
        if (times > 3) {
        let hello = 'say Hello instead';
        console.log(hello); // "say Hello instead"
        }
        console.log(hello); // hello is not defined
- - - - var greeter = 'hey hi';
        var greeter = 'say Hello instead';
        And
        var greeter = 'hey hi';
        greeter = 'say Hello instead';
  - - - let greeting = 'say Hi';
        let greeting = 'say Hello instead'; //
        error: Identifier 'greeting' has already been declared
    - - let greeting = 'say Hi';
        if (true) {
        let greeting = 'say Hello instead';
        console.log(greeting); // "say Hello instead"
        }
        console.log(greeting); // "say Hi"
        
        Why is there no error? This is because the two instances have different scopes, so they are treated as different variables.
  - - - const greeting = 'say Hi';
        greeting = 'say Hello instead'; // error: Assignment to constant variable.
      - const greeting = 'say Hi';
        const greeting = 'say Hello instead'; // error: Identifier 'greeting' has already been declared
    - - const greeting = {
        message: 'say Hi',
        times: 4,
        };
        
        greeting.message = 'say Hello instead';
      - const s = [5, 6, 7];
        s = [1, 2, 3];
        s[2] = 45;
        console.log(s);
        
        s = [1, 2, 3] will result in an error. After commenting out that line, the console.log will display the value [5, 6, 45].
        As you can see, you can mutate the object [5, 6, 7] itself and the variable s will still point to the altered array [5, 6, 45]. Like all arrays, the array elements in s are mutable, but because const was used, you cannot use the variable identifier s to point to a different array using the assignment operator.
- - - - console.log(greeter);
        var greeter = 'say hello';
        =》
        var greeter;
        console.log(greeter); // greeter is undefined
        greeter = 'say hello'; 提升申明先，然后才赋值
  - - - const must be initialized during declaration const j = 5;